1. Field of Invention
The techniques described herein relate to semiconductor devices and methods that can reduce leakage current and increase breakdown voltage for semiconductor devices. Such structures and methods can be used advantageously for devices having a compound semiconductor material such as a III-V semiconductor material, e.g., a III-N semiconductor material, such as GaN (Gallium Nitride), for example, formed over a second semiconductor material, such as silicon, for example.
2. Discussion of the Related Art
Improved power transistors are desired for advanced transportation systems, more robust energy delivery networks and new approaches to high-efficiency electricity generation and conversion. Applications of power transistors include power supplies, automotive electronics, automated factory equipment, motor controls, traction motor drives, high voltage direct current (HVDC) electronics, lamp ballasts, telecommunications circuits and display drives, for example. Such systems rely on efficient converters to step-up or step-down electric voltages, and use power transistors capable of blocking large voltages and/or carrying large currents. In hybrid vehicles, for example, power transistors with blocking voltages of more than 500 V are used to convert DC power from the batteries to AC power to operate the electric motor.
Conventional power devices (e.g., transistors or diodes) used in such applications are made of silicon. However, the limited critical electric field of silicon and its relatively high resistance causes available commercial devices, circuits and systems to be very large and heavy, and operate at low frequencies. Therefore, such commercial devices are unsuitable for future generations of hybrid vehicles and other applications.
Nitride semiconductor devices have been proposed as offering the potential for producing high-efficiency power electronics demanding high blocking voltages and low on-resistances. Nitride semiconductor devices have been formed of semiconductor materials such as gallium nitride (GaN) and aluminum gallium nitride (AlGaN). Nitride semiconductor materials may be epitaxially grown on various types of substrates. SiC (Silicon Carbide), sapphire and Si (Silicon) are the three most widely used substrates for the epitaxial growth of GaN. Each of these substrates has advantages and disadvantages.
SiC has the lowest lattice mismatch with GaN, and thus GaN grown on SiC has the lowest dislocation density and highest quality compared GaN grown on Si or sapphire other substrates. Highly resistive SiC substrates provide good electrical isolation. SiC substrates are also excellent thermal conductors which facilitates extracting heat from GaN transistors. However, the drawbacks of SiC substrates include a limited available wafer size (up to 4 inches) and high cost.
GaN grown on sapphire substrates has been used for the production of GaN-based LEDs (Light Emitting Diodes). Two-inch and three-inch GaN/sapphire wafers have been used by many LED manufacturers, while the usage of four inch-GaN/sapphire wafers has started to increase recently. Six-inch sapphire substrates are also expected to be adopted soon despite their higher cost. However, the major drawback of sapphire substrates is their poor thermal conductivity. In power electronics applications, the increased difficulty of thermal management associated with sapphire substrates makes them less favorable than other alternatives.
Si substrates provide a low cost solution for GaN power electronics. Four-inch and six-inch AlGaN/GaN-on-Si wafers are commercially available. Compared to sapphire substrates, Si also has higher thermal conductivity. Due to the low cost and good thermal conductivity of Si substrates, GaN/Si wafers have become the most popular platform for GaN-based power electronics.
However, the challenge of fabricating GaN transistors on Si substrates is their high leakage current and lower breakdown voltage than in GaN grown on SiC due to the higher conductivity of the Si substrate. Several approaches have been reported to increase the device breakdown voltages. One technique involves increasing the epitaxial layer thickness. However, the limitation of this method is the increased wafer bow with thicker epitaxial nitride layers. The increased wafer bow makes it difficult to fabricate transistors on large GaN-on-Si wafers. Another technique is to use substrate removal and wafer transfer technology, which may increase the breakdown voltage above 1,500 V, even with thin buffer layers.